1. Field of the Invention
The invention relates to a process for producing coated catalysts for the gas-phase oxidation of C.sub.4 -hydrocarbons to maleic anhydride, and also the use of these coated catalysts in processes for the gas-phase oxidation of saturated or unsaturated C.sub.4 -hydrocarbons to maleic anhydride.
2. The Prior Art
The preparation of maleic anhydride (=MA) from C.sub.4 -hydrocarbons by means of catalytic gas-phase oxidation has been known for about 20 years. Catalysts based on vanadyl phosphates (vanadyl pyrophosphate, vanadium-phosphorus oxides) are used in this process. The preparation of these vanadyl phosphates proceeds via a catalyst precursor which is obtainable by two reaction routes. The first is preparation in an aqueous medium; the second is preparation in organic solvents. The precursor is then, in a second step before or after shaping, converted into the actual catalytically active substance either in the reactor (in situ) or externally.
The reaction of the C.sub.4 -hydrocarbon in contact with the catalyst is carried out in various types of reactors. Examples include fixed-bed, fluidized-bed and also riser reactors, always using the abovementioned catalysts. For fixed-bed reactors, use is made of unsupported catalysts, i.e. catalysts which consist exclusively of the catalytically active component. These unsupported catalysts have the disadvantage that large amounts of active material have to be used. Due to the usually compact form of the shaped bodies, there is a high pressure drop in industrial reactors, which leads to increased energy consumption. These disadvantages can be overcome by the use of coated catalysts. Furthermore, the smaller amount of material in the case of coated catalysts results in a significantly reduced local temperature increase in the catalyst. This has a positive influence on the selectivity of the catalyst. Since the active component of a coated catalyst is present only in the outer shell on an inert support, there is a significant reduction in the amount of material required. Selection of catalyst supports having a suitable shape (e.g. rings) reduces the pressure build-up in the reactor. This reduces the power required for the gas blower.
EP-A 72381 describes a process for producing a coated catalyst for the gas-phase oxidation of C.sub.4 -hydrocarbons to MA. To prepare the precursor, a solution of V.sub.2 O.sub.5, uranyl acetate and H.sub.3 PO.sub.4 in isobutanol is heated in the presence of hydrogen chloride. The resulting precursor is isolated by distilling off water and isobutanol. After drying, the uncalcined precursor is applied to inert supports. The application is carried out by moistening the inert support material with water in a first step. Subsequently the precursor is applied, possibly in small amounts, if necessary with further addition of water, without use of a binder. Due to the initial moistening of the support with water, a porous material having a certain water absorbency is used. According to this document, contents of applied material of from 50 to 80% by weight, based on the weight of the coated shaped body, are necessary for producing usable catalysts. The activation or calcination of the material obtained in this way is carried out at a temperature of 400.degree. C. for 16 hours in air after application of the precursor to the support body. Due to the application of the uncalcined precursor in aqueous medium, the catalyst activity is unsatisfactory.
WO-A 96/25230 describes a process for producing coated catalysts for the synthesis of MA, in which the precursor is not mixed with water. Instead the precursor is mixed with an organic solvent and optionally with a binder and is then applied to the support. Finally, the supported catalyst is converted into the catalytically active form by calcination. This calcination can be carried out either in the reactor (in situ) or externally. The catalyst produced in this way contains from 0.02 g/cm.sup.3 to 0.4 g/cm.sup.3 (bulk volume) of active component. The thickness of the active component layer is from 0.1 mm to 0.8 mm. It was found that application of the uncalcined precursor in an organic solvent gives more active supported catalysts than when the application is carried out in an aqueous suspension. However, a disadvantage is that use of an organic solvent leads to problems because of the explosion risk posed by air/solvent mixtures formed during evaporation.